The invention relates to a device and a method for identifying overlaps of flexible flat items being moved along a conveyance path in a conveyor segment which is configured to move the items sequentially, while they are standing on edge, and such that at least one section of each of the items is deflectable perpendicularly with respect to the conveyance path.
Devices and methods for the separation of flat items are known, for example, from European Patent 0 142 538. A problem in such separation devices is the fact that so-called double-drawing errors occur which are attributable to instances where, instead of one item being drawn from a given stack, two items are drawn simultaneously. This may occur because the frictional entraining force between the front item in a stack and the second item is too great, or because the second item in the stack projects from the stack and is drawn off too early due to the fact that the alignment of the front edges of the items is non-uniform.
Undetected instances of double-drawing are of special importance in letter-sorting systems because they lead to increased missorting rates resulting from reading errors made by the automatic address identification system or from keying errors when addresses are entered manually. In addition, overlapping items created by double-drawing can increase the susceptibility of letter-sorting systems to mechanical failure. By treating identified double-drawings as reject items, however, missorting and mechanical failure can be avoided.
Problems caused by double-drawing errors can in addition be reduced by identifying item overlaps. In order to identify item overlaps, optically scannable track markings, have, for example, been applied to item surfaces. Lines, bar codes, or other patterns can be rolled or sprayed, preferably over the entire length of the item, on either its front side or its rear side or both in order to create these optically scannable track markings. Item overlaps can thereafter be detected by optically scanning these track markings because, where overlaps exist, certain patterns, such as graduation marks or lines, are usually missing or skipped. In such processes, it is necessary to pretreat the items in two general steps. While the first step involves the marking of tracks on the items, and occurs after a first item sorting the second step involves the actual identification of item overlaps, and occurs only after a second item sorting stage (for example after the detailed final sorting of letters). A clear disadvantage of the above process is that any missorting due to an overlapping of the items caused during the first item sorting stage could not be identified. Another drawback of this process is that relatively large forces are necessary for the flexing or bending of items so that possible track interruptions can be detected. Yet another drawback is that track markings may only be applied to those regions of the items that are not covered by conveyor belts, in order to prevent smudging of the surface of the items and the conveyor belts.